This invention relates to coating compositions based on aqueous dispersions of epoxy resin-derived polymers, the compositions having particular utility in the formation of protective coatings for food and beverage containers and other metal substrates.
A variety of coating compositions have been formulated particularly for use as internal coatings for beverage containers and especially for beer cans. Such coatings must be continuous (that is, pin hole free) to prevent contact of beverages with metal container surfaces, must be inert to attack from the beverage ingredients even at elevated storage temperatures, must not contribute to (or subtract from) the beverage flavor, and must be capable of withstanding sharp bends and impacts as beverage cans are dropped, dented or otherwise deformed during handling. Although polymeric coatings may be formed as solutions in organic solvents, it is generally preferred to avoid as much as possible the use of organic solvents which are costly and environmentally undesirable and to employ either 100% solids coating materials or coatings comprisinq polymeric species that are dispersed, as latexes, in aqueous media. Coating compositions based upon epoxy resins have been particularly valuable inasmuch as such compositions provide coatings characterized by excellent impact resistance, barrier properties and resistance to beverage ingredients. Epoxy resins, however, are relatively expensive; as a result, attempts have been made to add somewhat less expensive polymeric components, such as styrene and (meth)acrylate addition polymers. Examples of coating compositions of this type are found in U.S. Pat. Nos. 4,285,847 (Ting) and 4,446,258 (Chu and Spencer).
U.S. Pat. No. 4,285,847 refers to an epoxy acrylic polymeric surfactant made by free radical grafting of ionizable acrylic side chains to an epoxy backbone at points of hydrogen abstraction followed by neutralization and dispersion in water. A second polymer is emulsion polymerized in situ in the epoxy acrylic surfactant dispersion to form a separate polymer phase. Cost reduction is obtained from the less expensive second polymer which permits the use of less of the expensive epoxy component without compromising required film properties.
In U.S. Pat. No. 4,446,258, a different type of epoxy acrylic polymeric surfactant is made by esterifyinq an epoxy resin with a preformed ionizable acrylic polymer and then dispersinq the composition in an aqueous vehicle. A second, low cost polymer is then polymerized in situ. Additional benefits are obtained in this procedure in the latitude of solids and viscosity that can conveniently be obtained and from the modest amount of expensive amine neutralizer that is used to obtain stable water dispersions having useful viscosities.
Useful compositions of the type described in the above patents may contain up to about 25-30% by weight of low cost monomers such as styrene, and usually require in excess of 50% by weight of the expensive epoxy resin. Attempts to use greater proportions of low cost monomers usually result in the loss of the combination of flexibility and barrier properties needed for can liners.